Air filtration is used in a wide variety of environments to remove pollutants in an attempt to purify the air. Though there exist several types of air-filtration technologies such as mechanical filters, frictional electrostatic filters, and electric filters, active electrically enhanced air-filtration systems have become increasingly popular because of their high efficiency.
Although numerous publications and patents exist for electrically enhanced filtration using a corona pre-charger, less information has been published on the relative importance of the various electrostatic collection mechanisms acting within such filters. A conceptual diagram of a prior art electrically enhanced fibrous filtration system is shown in FIG. 1. The electrically enhanced fibrous filtration system includes an upstream metallic screen through which air enters the filter, a pre-charging unit downstream from the screen and a filter downstream of the pre-charger. The pre-charging unit may be provided with bi-directional corona discharges as shown. As uncharged particles enter the filtration system, the particles get charged by the electric field created in the space-region between the pre-charger unit and the filter. The filter is electrically enhanced in that it is actively polarized so as to attract charged particles to portions of the fibers with a charge opposite that of the particles.
Typically, particle and fiber interactions are mainly explained in terms of three electrostatic forces of attraction: a) polarization force due to dipoles induced within the filter fibers and/or particles by external electrical field; b) image force due to induction of charges on uncharged fibers by charged particles; c) columbic force due to charged particles and polarized or charged fibers. The electrostatic interactions are discussed primarily in terms of the gross parameters of electric field strength across the fibrous media and particle charge. The concept of unipolar ionic emission to charge the filter fibers and enhance the collection efficiency has not been successfully employed. The unipolar ionic fiber charge, originating from the corona discharge electrode and accumulated on the filter fibers, produces a non-uniform electric field around the fiber which causes a force of attraction between the charged fiber and both oppositely charged and neutral particles. In the case of a neutral particle the attraction is caused by induction of a dipole within the particle.
In light of the foregoing, improvements continue to be sought for electrically enhanced air filtration devices.